In power transmission systems, synchronous generators are connected to transmission lines through step-up transformers and are brought up to a rated frequency and voltage by their respective turbine and excitation controls. In the steady state, the frequencies of all the generators in the system are identical and the phases of these generators are constants, thus providing constant power and reactive power outputs.
A major disturbance such as a fault in a transmission line can force one or a group of generators to lose synchronism. When one or a group of generators is forced out of synchronism by the fault, the one or group of generators must regain synchronism with the power transmission system. A generator losing synchronism may lead to high peak currents, pulsating torques, high rotor iron currents and mechanical resonances that are potentially damaging to the turbine-generator.
Sometimes the affected generators are able to regain synchronism successfully, but sometimes they fall further out of synchrony. Such a generator may need to be tripped and isolated if other actions to retain system integrity fail.
If no corrective action is taken, there can be further separation among the affected and unaffected groups of generators. Even when only one generator loses synchronism, allowing it to continue operating unsynchronized can result in serious damage to the generator due to high amplitude currents and pulsating torques. There is also risk of losing the auxiliaries of the unstable generator along with those of nearby synchronized generators.